Can we use encrypted apps like WhatsApp/ Signal for discussing operational, PII, etc. purposes?

It’s a good question and one asked or considered often I’d bet.
But ask yourself
Who controls the data ?
Is data sold?
DATA may be encrypted but that also means make an unencrypted

Good rule of thumb if you would not say it standing in line at Starbucks don’t say it on other than an appropriate DOD Communication asset

Hey there! Using encrypted apps like WhatsApp or Signal can be a good option for discussing operational and PII matters. Both apps prioritize user privacy and employ end-to-end encryption. Signal, especially, has gained recognition for its security features. It's up to you to decide!

Using encrypted messaging apps like WhatsApp or Signal for discussing operational matters or Personally Identifiable Information (PII) depends on the specific policies and regulations of the organization or institution involved.
https://whatsmb.com/gbwhatsapp-apk-download/

AB (Join to see) - Actually: "In accordance with DoD Instruction 5200.48, DoD personnel will not use non-DoD accounts or personal e-mail accounts, messaging systems or other non-public DoD information systems, except approved or authorized government contractor systems, to conduct official business involving CUI. In accordance with DoD Instruction 5200.01, DoD personnel will not use unclassified systems, government-issued or otherwise, for classified national security information. DoD CIO will continuously consult with all DoD Components to evaluate risks mobile applications may present to the DoD and update References (m), (n), and (o), as appropriate. "

So technically no one should be using Signal or WhatsApp to push anything CUI. Do units do it? Most likely.

Thats what I got as well in AR 530-1; 2–18, (15)