Meeting room response message shows as plain text in outlook.

Hi everyone,

We recently encountering a problem described as below.

1. a meeting room resource setup.

2. after we sent a meeting request to the meeting room, we can receive the request accepted like below.

but the content shows as plaintext (I already setup additionalresponse like "<font color=red>TEST</font>", anyway the words should have color)

after all, i found everything in OWA is fine.

So, i want to know why this happened? and any setting on outlook and exchange server can fix this? thank you