This happened to me this morning but I wasn’t in a position to write it up at the time, hopefully this might give you something to go on looking through any logs.
I went to invest in my ISA pie and did a custom distribution of a 50/50 split between two of the elements in the pie. Everything appeared like it worked, i.e there was no indication anything had failed and no error dialogs, but the whole amount just ended up as spare change in the pie, and checking my history no market orders were attempted.
I thought I had just pressed cancel or done something silly so I withdrew the free cash from the pie and tried again and the second time it did the same thing and ended up as free cash in the pie again.
I then tried a third time (this time I accidentally put a couple of quid extra in the deposit) and followed the same process, which worked successfully and I ended up with 2 market orders as expected.
Probably not relevant, but for the sake of completeness the cash I added to the pie was all from free funds in my account and not a deposit. The first attempt (before I withdrew the free cash and started over) I’m pretty sure there were a few pennies in the pie as spare change as well.
Don’t know if it was something like the amount I tried to invest wouldn’t split equally between the 2 pie elements at current prices, but if that were the case I would expect 2 orders to have been placed and a few pennies left over as spare change. In any case there were no error messages, and had I not been paying attention I would have thought the amount had successfully been invested.