Consider upgrading to a current release. There is a transcoder problem that was fixed in 1.18.7.2415. The problem affects Apollo Lake CPUs, which is the type in your NAS (Celeron J3455).
(Transcoder) Hardware decoding could produce corrupt video when burning subtitles on Apollo Lake-based processors. (#10753)
As a test, disable hardware accelerated transcoding and see if the problem disappears. The video may buffer, as the CPU will have trouble keeping up, but it should be clear.
The current public release is 1.19.3.2764. The current beta release is 1.19.3.2793.
