I can repro this too.
If you change
AND T2.INVENTSERIALID = T1.INVENTSERIALID
AND T2.ITEMID = T1.ITEMID
To
AND T2.ITEMID = T1.ITEMID
AND T2.INVENTSERIALID = T1.INVENTSERIALID
It is able to produce the desired plan.
Somewhat unsatisfying that you have to do this but previously with MERGE join and MERGE union I have also found that tweaks are needed to get the hoped for plan (example).