Geoserver uses print-lib-2.1.2.
A bug was detected on this version, using PDF templates on background.
Each PDF opened is never closed.
The fix is done at version mapfish-print 2.1.x, but wating for a 2.1.3 release to purpose a PR on geoserver.
Version 2.1.3 of mapfish-print released:
PRs merged into master and 2.14.x.