When you have a plitter between a and c and c has the power block and a is not used, only c is used... the lnb does not receive power when A does not deliver power. So A need at least to deliver power continuously... (or you have to add a power inserter) as far I remember vu delivered in the past continuous power and now they don’t do that anymore... and/or A delivers the incorrect power what C needs...
An active tuner always supplies power. That's where your reasoning is wrong. There was a setting on older STB's to (in)activate power output from the tuners, but that's only considered when they're not used. An active tuner ALWAYS supplies power. See my explanation earlier.