Gadol, no one at any time ever said here that “Rabbonim monitor those who have been megayer and withdraw their geyrus if the gerim go OTD.” Of course not. If a Ger c”v goes OTD he/she remains a full-fledged Jew forever. The issue here is if the statement accepting kabbalos ohel mitzvos during the conversion process was insincere, then the conversion was never valid and completed in the first place and the person never became a Jew.
As you yourself now conceded and acknowledged as possible in your most recent comment. Except that this principle doesn’t only apply to situations of “block conversions” , such as with the Russians, but rather the principle is true and applicable in any and all conversions, including individual ones, when the person was insincere regarding kabbalos ohel mitzvos.