If it happens again, try just disabling and re-enabling the peer to see if that fixes the advertisements.
In my cases where this bug occurred, only disabling and re-enabling the whole bgp instance would stop those prefixes from being advertised.
This bug exists since RouterOS dropped Quagga and implemented their own BGP. So well over a decade...
It was as if the rule had been "cached" and not refreshed.
There's a bug with the route filters list where you need to disable/enable the filter in question to get it working. This is especially prevalent when creating new filters. It's been this way since forever, to the point that it has become muscle memory nowadays to disable/enable every rule I touch in route filters.
Edit: actually only enabling (even an already enabled) filter makes it work also.