Thanks. Don't suppose you can think of anything else to try?
Well... so far, according to other members, a restart, soft reset, hard reset, re-installation of apps and re-format using "Nokia Care Suite" have already been done, but mostly with no good results. I could not test all of these and see which one works (because my Messenger and WhatsApp are currently working normally in the first place), but these are the general methods of repairing or addressing bugs with most electronic devices.
Since none of those worked, what's left is to
send useful and constructive feedback to the developers of these apps. In Messenger, there is an option to "Report a Problem" in the app settings. In WhatsApp, there's an option to "Contact Support" in the app settings. This presumes that the developers receive (and they do) the feedback and implies a waiting period for the developer to issue a fix through an update.
Of course, one could try using a different phone but that may or may not increase the statistical chance of not experiencing the bug.