this happens with demand as well, though it's harder to reproduce and is unlikely to happen in normal gameplay.
to reproduce, either in creative mode or in normal gameplay:
1) create a two power generators with equivalent power generation capacity
2) connected them both to a main network through separate transformers. make sure the transformers are off, and the generators are ready to produce (and would produce if there was demand and the transformer was on)
3) create a demand sink
4) connect it to the main network through a transformer. make sure this transformer is on.
5) turn on one supply transformer.
6) turn on the other supply transformer.
expected:
supply balances evenly between both supply transformers according to the usual factorio behavior (all suppliers supply at the percentage capacity equivalent to [Production in MW]/[Satisfaction in MW])
actual:
the supply generator turned on first retains high usage even when the second supplier transformer is turned on.
workaround (uncertain how practical this is in a real game scenario):
turn off and on the first transformer. this seems to cause the second transformer to finally update, and once the first is turned back on, the suppliers balance out the load.
notes:
i expect this has to do with the internal buffer logic and power transfer logic not updating when new transformers are turned on when power satisfaction is already at 100%. i.e. the transformer logic "thinks" that no more power needs to be transferred (through the logic related to the buffer size maybe), so it doesn't change the buffer and thus doesn't balance the power load across both suppliers. however i'm not certain that this is the case since i don't fully understand how EEIs and their buffers work.
