EFI partition doesn't show up after creating USB boot

I've checked the docs and posts but couldn’t find an answer, so apologies upfront if this is common knowledge.

I have a mid-2011 iMac running High Sierra and I'm trying to install Sequoia with OCLP 2.2.0.

I can build and install OCLP on my USB drive without issues, and the EFI partition shows up during the process. However, as soon as it finishes, the EFI partition disappears—it doesn't appear in Startup Disk or Disk Utility—so I can't proceed with the installation process.

Holding the Option key on restart doesn’t bring up the bootpicker screen. The EFI partition only shows up if I manually mount it using Terminal, but it hides again after a restart. I can also only access recovery mode via Terminal; holding the keys on reboot doesn’t work either.

Any ideas on what might be causing this? The USB drive is relatively new (about a year old) and has been formatted using the recommended settings in the documentation.

Thanks in advance!