Ubrokeifixit Galaxy Watch Back Rear Lens Glass with Tape Replacement for Samsung Galaxy Watch 46mm SM-R800 R805/Galaxy Watch 42mm SM-R810 R815 Black(Rear Cover NOT Included) https://www.amazon.com/dp/B07W987BLV/ref=cm_sw_r_apan_glt_fabc_95A4S10YQJVADC6T0DSM?_encoding=UTF8&psc=1
Just ordered one to replace mine along with the HR monitor.
If Samsung refuses to repair it under warranty, you can order new sensor glass with adhesive and try to repair it yourself. Somebody in this sub wrote about that. You can find those glass on Amazon or eBay.