Like many on here I had been having many of the same issues, missing all sorts of notifications, not just text since upgrading to ios 14 (first 14.1 then 14.2). Here is what fixed my issue... I went to settings->notifications->messages (for this example) and scrolled down to the options section. For the show previews section the "when unlocked (default)" was selected, meaning only notifications would only appear when the phone is unlocked when is hardly ever the case. I switched it always and the issue is now gone. I had to go into may other apps one by one and toggle this option back to always. Showing previews only when unlocked was 100% not my setting before this update.
What is interesting/****** me off is my wife just upgraded to ios14 and did not have issue. I went into her phone to settings->notifications->messages and for her show previews option was selected as "always (default)". Why on her phone she has options listed as "always (default)" and on my is "always" and "when unlocked (default)" I have no clue.
Thanks Apple!