Update: This problem is now fixed. The PCManFM for raspbian buster has been updated to v1.3.1-1+rpt18
Update: This symptom seems to be caused by RPI Foundation. See https://www.raspberrypi.org/blog/a-new-raspbian-update/#comment-1521860 I will update this post when the fixes are applied.
Filename with SPACE appears as '%20' on SMB/CIFS shares