sp_releaseschemalock is an undocumented external proc which simply releases the schema lock:
As per our past experience on this, we witnessed the same but never had the issue with
sp_releaseschemalock being a culprit.
Kindly check the complete details when checking the blocking. That is ,you should not completely ignore the one that is getting blocked rather than completely dependent on blkd_by_spid, as blkd_by shows sp_releaseschemalock as culprit.
Also would like you to refer this
You should check your linked server configurations and make sure they
are "collation compatible" checked. Otherwise the entire remote table
is returned to the local calling server to execute the WHERE clause.
As mentioned, you need to go through the settings for the query getting executed via linked server because the primary server wont have the required statistical information when fetching the same from other server through its linked query.