found a bunch of f6t5d blister packed for dorcy.they were all falling apart so i emptied them all into a box.some rattled so i tested them.most started dim,ramped up to brighter than fully warmed up,then settled down again.why would they hit a point where they were brighter than when fully warmed up?
test rig is a choke with push switch to start.tubes are moolim brand.

You will have to operate them about a hour for the internal "chemistry" to really settle down (due to low pressure you rely just on the diffusion to distribute the materials inside the tube) and so be able to take some relevant measurements.
It could be the mercury getting rather quickly released from the electrode assembly (lead in wires,...), but after some while settling on the cold tube walls.
It could be the arc voltage was temporarily higher, what mean higher real power (more than the rated 6W) coming from the ballast during that period.
But both are just speculations, it could well be something totally different. The only thing what remain valid is the need for the ~1hour time till the lamps recover from the storage and stabilize so, you can assess their performance.

I use to have a Moolim F4T5/D, and it showed similar behavior when I tried it on my preheat strip light. It would have trouble starting, and once it started, it would strobe and swirl quite violently, and eventually settle down after a minute or so. However, it would work absolutely perfect on an electronic ballast.

I currently have two generic F4T5/D lamps that came out of a Coleman lantern, and like the Moolim, they also act weird on a magnetic ballast, but run flawlessly on an electronic ballast. I also noticed that they were much brighter than a couple of GE F4T5/D lamps I have, and when I measured the arc voltage, it was around 55 volts - almost that of an F8T5!

i know there is another box of these in the shed.will fire a few up with the killawatt in line and watch during startup.

Some bulbs have troubles starting or warming up from being stored vertically, the Mercury migrates south.