What does it mean if my VOA extension stamp says I need a re-entry permit before leaving Thailand?

Just never overstay, not even 1 day

They let me leave, but then made me submit one when I came back into Thailand

it is funny, they took the same stamp they normally use for the "1-year Extension of Stay" based on marriage or retirement. I guess they ran out of other (smaller) stamps. . . . . So you got stuck with the reminder that you can get a re-entry permit, but given that it would only keep a 30 days stay permit alive, gosh, it is not really worth to buy one

You cannot extend a VOA (Visa on arrival, you have to apply and pay for at entry) by 30 days, the maximum is 7 days. You were on a "visa exempt"... completely different thing...

To the re-entry-stamp, yes... that's totally normal. Nothing to worry about, no "must" to buy one.

Yep,

it's just a friendly note from immigration that if you are planning to leave and going back to Thailand before the permission of stay date expires, and want to use the remaining days left, you need to buy a re-entry permit. They stamp it usually to remind those who has 1 year extensions and sometimes they leave the country and forgot to buy the re-entry permit and when they return, their extension of stay is void already.