Never had this before. Fluorescent tubes in my experience either work at
full brightness, flicker, or just the heaters glow.

I had a flickering tube, installed a new one, and it started fine but was
noticeably dimmer in the middle than at the ends. When I got it out of the
store, one pin had been bent, which I bent straight and I tested the heater
at that end with a meter - it was ok. Now obviously it's had a clunk to bend
the pin. If it had spoilt the seal and made a 'soft' tube I'd expect the
heaters to glow brightly then burn out which it didn't. So what process
makes for a dim tube? End 25% at both ends is normal brightness. Could the
phosphor have been dislodged in the middle but not the ends?

I'll leave it in place and observe it's progress as where it is is over a
shelf anyway.

Andrew