Your version of Office Online Server is quite out of date, but unless you see patch notes between Jan 2019 and now that resolve this specific issue, I would do two things:
1) Patch to October 2020 first.
2) Open a support ticket with Microsoft.
OOS (nor Word on the web for M365) doesn't have perfect Word doc compatibility. Reporting these issues through a support case might lead to a resolution if it hasn't already been resolved by a previous patch.