The TypoScript condition:
page.5 = TEXT
page.5.value = NOT LEVEL 0
[tree.level == 0]
page.5.value = LEVEL 0
[END]
Sometimes it results in "LEVEL 0" on subpages and sometimes in "NOT LEVEL 0". I can't recognize a system, it changes sporadically. But it seems to keep the value after a reload and may change it after visiting the root page and clearing the FE cache in between, even though caching is actually disabled:
page.config.no_cache = 1
I thought it could have something to do with that my condition may be wrong (maybe 0 is sometimes interpretet as "false" instead of zero), or a TYPO3 Bug.
In TYPO3 v9, new conditions based on the symfony expression language were introduced. Some of these conditions are not yet working reliably and have to be fixed – especially regarding the rootline/page tree.
Your problem seems to be related to this issue: https://forge.typo3.org/issues/86837