This question arises because of the different understandings of license validity in Commission Regulation (EU) No 1178/2011 (Aircrew) and No 1321/2014 (Continuing Airworthiness).
In Reg. (EU) 1321/2014, the pilot-owner authorisation described in M.A.803 assumes that a pilot has sufficient technical knowledge to perform certain maintenance tasks. While exercising such pilot-owner authorisation, the pilot-owner even further develops his/her competency in maintenance. Hence, in the case where the medical examination has not been conducted or not been passed and the licence has therefore lost its validity, it is the intent of the rule to allow the pilot-owner to continue using this authorisation as long as he/she still considers himself/herself physically fit (including good visual acuity) and competent to carry out such maintenance (ref. point (a)(2) of Appendix VIII to Part-M).
This is the reason why a new point (5) was introduced in AMC M.A.803 in 2016 (ED Decision 2016/011/R) stating: “not holding a valid medical examination does not invalidate the pilot licence (or equivalent) required for the purpose of the pilot-owner authorisation”.