Hmm yes interesting find. Strange that it is missing from the Japanese DB (Tagoo).
Looking around I do find that there was a polar star 2 released for the Sharp X1 and the MZ2000.
The Japanese title is
ポーラスターＩＩウィルヴィドウ
On the screenshot it says (c) MICOM HOUSE followed by SPS. In the text next to it, it says マイコンハウスSPS
I think those are two companies, one is the developer and the other the publisher. Neither is listed here on GenMSX, but I can find SPS on Mobygames.
I am having trouble making out what it exactly says in the title bar written in black, but it basically says something like "scheduled to be released".
There is some discussion about it in Japanese here:
https://game12.5ch.net/test/read.cgi/retro/1179413219/ but for a non-Japanese it is very difficult to follow using google translate.
They also mention it being marketed as the "industries first 4Mbit ROM".
In any case, it does seem like it was never released.