I did similar to you, with a full format of the JFFS partition and then a full reset. Manually entering my details was simple, I merely use a VPN client. However wireless client dropouts happened enough for me to be concerned, on devices that had previously been very stable. Nothing was showing to be an issue in the logs, well nothing that I could recognise. Either a reset of the router or making changes to the affected radio did resolve those., temporarily.
Some of those wireless issues were noted on the 2.4Ghz radio and not just on the 5Ghz. I didn't always need to reset the router to resolve the drop and failure to connect, for some devices, just changing the channel could have the same effect for them to be able to connect again. I suppose that is similar to a reset, in terms of the wireless connection.
Going back to 384.18 and all has remained well, showing around 40 clients or so connected and, as yet, no unusual behaviour.
I have had this 86U router for almost three years and this is the first time that I had needed to "downgrade" a Merlin release in all of that time, used these firmwares since bought.
Lazy maybe but it was also the first time that I had factory reset and "cleansed" the router..!