This is really may not be a problem with subsyncarr....but after adding the alass engine some subs don't get generated in which the other engines do generate there sub versions. I went into the container and ran the alass command from the command line and it complains that it can't parse the input sub.
Going to the alass github it shows that a previous upgrade request to auto covert the input sub to UTF8 format was now incorporated into its latest.
So wondering if your container is using the latest alass? If so maybe alass still cannot parse non-UTF8 input sub files. If this is the case may be possible to convert the non-UTF8 using a set of cli commands like chardet to detect the sub file format and iconv to convert UTF8 prior to running alass as a workaround
TIA for any consideration.