I hope it's okay I am bundling these issues together, as I suppose they might be related.

1. /etherpad export endpoint always returns the head revision
   /p/{padname}/{rev}/export/etherpad
   for all 'rev' values.
   Unlike both the /html/ and /txt/ exports, which work as expected.
   This also affects (and can be reproduced via) timeslider's "Export current version (as Etherpad)".

2. On the API side however,
   getHTML(padID, [rev])
   ignores 'rev', always returning head revision.

3. ... while
   getText(padID, [rev])
   works as expected by taking 'rev' into account.
   But it has a different return schema when 'rev' is specified. It wraps the "text" object in another "text" object, and also adds an "attribs" blob.

Etherpad 1.8.13 + no plugins
Ubuntu 18.04.5 LTS
Node v12.22.1
npm 6.14.12