Quick update: the ‘restore bug’ is confirmed for the DS218+, using DSM 6.2.1-23824u4, Mono 4.6.2-0096, and Duplicati 188.8.131.52. Navigating/selecting files to restore indeed does not work when the path contains spaces. Restoring itself is not a problem though (when selecting the entire parent folder for restoring).
I have not been able to find any info on exactly how widespread the bug is. However, given the number of people that have reported it, I would not be surprised if it occurs on all Synology systems with (at least) recent versions of DSM and Mono.