I was adding all those member in 聖飢魔Ⅱ but at some point it became impossible to set instrument any more.

Strangely enough, it concerns all the following support members, the last I had input:

additional members:

• 怪人マツザキ様 (1987-02 – 1999-12-31)
• レクター・H伯爵 (1992-07 – 1994-08)
• 怪人マツザキ様 (2005-11 – 2005-12)
• 怪人マツザキ様 (2010)
• 石黒彰 (2015-08 – 2016-01)
• 怪人マツザキ様 (2015-08 – 2016-01)

They should all get Keyboard as (AC) support keyboard.

But it’s now not only them, try to Add relationship Artist and you will see no AC field right to the instrument selector and no instrument selection will be retained.

Actually I can no longer edit any instrument memberships, same at RX…