Any specific logs I can inspect?
I'm not very familiar with FreeNAS (since it's based on FreeBSD), but i would check directory
/var/log and check
syslog file.
cd /var/log
cat syslog | grep electrs
Sorry, but this is a question for the author to analyze his problem. I think there is a problem with the mempool
Why do you think the problem is related with Bitcoin Core's mempool?
I do not know about the author, but it happened to me when there are transactions in the mempool that were overwritten or with erroneous waste and the local mempool did not match the network one - I could not get data by addresses.