[mytailorisrich]

I have found this:

The Temporary Visitor Visa is for activities not exceeding 90 days as follows: tourism, visiting relatives or friends, recreation, convalescence, attending a conference, participation in unpaid lectures, meetings, amateur athletic meetings or other contests, short business trips (e.g. market surveys, business talks, after sales service for machinery imported into Japan)

This category excludes profit-making operations and paid activities. [1]

Usually, paid work means just that.

The thing of course is that you do remote paid work it is unlikely that anyone will notice, especially if you indeed only stay for a few weeks. Plus if business trips are ok, then I suppose noone will care.

[1] https://www.mofa.go.jp/j_info/visit/visa/visaonline.html