ipad pro battery

Hi. Got a really strange issue with an iPad Pro 2018 11” (purchased December 2019).

Battery life is normal (good) when in use or on standby. But in the last two weeks something strange has started happening: when the battery got low (around 10%) I charged it to 100% then swithched the iPad off (completely off). 4 days later, try to switch on, but battery is completely dead. Connect to charger and it charges at normal speed back to 100%. Switch off. 4 days later switch on, zero charge again! Battery monitor shows battery health is normal, and in use there is no problem with battery life. But it won’t hold its charge when turned off even for just a few days. Apple support have run diagnostics and can see nothing wrong, and they have never heard of this issue before. I haven’t installed anything new recently, nothing has changed on the iPad, and it was behaving perfectly until this issue arose. Anyone got any ideas or experienced the same problem, before I have to send it back to Apple?

Thank you.