That is putting it mildly. Things have been in flux since 2Aug18, when Amazon began enforcing the “Search Conformity Initiative” (announced in that day’s News Item “Update to search term field” {link}) that limited Search Terms to 249 bytes (not characters) and stopped excluding spaces between keywords, to boot. The two more-recent initiatives in Sponsored Ads/Sponsored Products/AMS (the un-announced revamp of Campaign Manager, and the revamp announced in the 5Sep18 News Item “Introducing Amazon Advertising” {link}), combined with earlier initiatives on the brand-policing/PDP Title & PDP Image fronts, have converged with the increasing frequency of problems with Manage Inventory - the key database to which most everything else about an Amazon listing is tied - to produce an unholy mess.

The scenario you describe is, by and large, indicative of the variances inherently-required in creating code for different browsers, and for different devices - but there can be little doubt that despite Amazon’s long-time lack of adherence to Best Practices in that regard (partially due to its silo-management model resulting in one hand not knowing what the other is doing, but primarily due to the pursuit of protecting its proprietary information and preventing bad actors from gaming its systems, as @rms astutely points out upthread), it for long maintained an ability to keep search results fairly consistent, and at least fairly-relevant.

This is demonstrably no longer the case at the current time, as evidenced by the lack of indexing for various portions of previously supported Search Fields such as Product Titles, Bullet Points, Product Descriptions/EBC/A+ EMC - a lack which has continued to be both sporadic & intermittent since 2August, which suggests that the engineers & technicians are working furiously behind the scenes to fix what the programmers broke.

Until the dust settles, and Amazon is able to return things to the status quo ante (not necessarily a given, unfortunately, for a variety of reasons), it will prove exceedingly difficult for 3P Sellers to determine a course for moving forward - and it should be remembered that anything we do in terms of changes introduce new variables into the already-complex equation, which the folks undertaking repairs must thereby take into their calculations. Still, there are some things you can do to try and improve your chances of making consistent sales.

The first thing I would do in your case is to check the Search Terms field (the ‘keywords box’ in Manage Inventory Editor), and ensure that what you have is in comportance with the new requirements, which are explained in the Seller Help Content’s Using search terms effectively (link). The built-in byte counter for that field was enabled with the new initiative, but Amazon has not seen fit to provide blanket notification to all sellers in Manage Inventory (& often enough, not in Manage Inventory Editor, either) when the 249-byte limit is surpassed. You can use the 3rd-Party Byte Counter that Seller Support uses, Lingojam, to help with doing this; there’s a direct link to it included in my posting of a Case Log Response we received for an inquiry made about spaces no longer being excluded from the count in this 8Aug18 reply in another thread:

Once you’re sure that your listing(s) Search Terms field is being correctly indexed by Amazon’s A10 Search Algorithm, I’d make sure that everything else comports with published policy as embodied in such Seller Help Content as Product detail page rules (link), Optimize listings for search (link), and Help Customers Find Your Products (link). When you’re satisfied that all your ducks actually are in a row, check the indexing of the aforementioned Search Fields (c: 3rd ¶), and adjust accordingly - there’s likely to be some degree of split-testing required to reach the sweet spot if you find anything is not being indexed after you’ve undertaken all of the above, but I would recommend not rushing through that process, so as to make sure that you’re not seeing the intermittency (i.e., some days a certain thing is indexed, the next day it’s not, and vice versa, etc.) that so many of us are continuing to see.

Good Luck - we may all need a healthy dose of that before the dust settles.