Did you try turning off page caching on the org (in Session Settings, scroll up and uncheck "Enable secure and persistent browser caching to improve performance" in the "Caching" section)? I know you mentioned clearing the cache though it isn't clear which cache you cleared. –
May 12 '21 at 7:18
Posting Phil W's response as Answer because in 12 years of SFDC, I hadn't ever seen this occur but Phil W's solution resolved my issue. My FullSB change set was clean (fls, accessibility, profiles, etc. all there) and there was space on the page for the fields, they just weren't there in LEX. Cleared my browser cache but disable caching on session settings made the fields visible. When I re-enable it, the fields disappeared again. Disabled it is. Original poster never followed up, but this solution works for one use case of invisible fields.