hashing passwords/passphrases

I presume you observed a string like "_e4h9VgHA`JG1MSE@c.e?2lkn>ZG[Wombh[o>67kaC_kV`J" (not exacly 50 characters but close enough). BIG-IP started to experience the issue since 14.1.0, however, I did not see the behavior on 14.1.2 (Final). F5 identified the issue but no fix is provided yet.

Meanwhile, you can use the /mgmt/tm/util/bash endpoint to directly evoke tmsh command. For example, getting the secret property from RADIUS setting (equivalent to 'tmsh list auth radius-server system_auth_name1 secret') using curl:

# curl -sku $PASS https://$HOST/mgmt/tm/util/bash -X POST -H "Content-Type: application/json" -d '{"command":"run", "utilCmdArgs":"-c \"tmsh list auth radius-server system_auth_name1 secret\""}' 
{
    "command": "run",
    "commandResult": "auth radius-server system_auth_name1 {\n    secret $M$Ti$Jh/EsK9GLFOp02s5m8kgjg==\n}\n",
    "kind": "tm:util:bash:runstate",
    "utilCmdArgs": "-c \"tmsh list auth radius-server system_auth_name1 secret\""
}

Not elegant, I agree (especially with the quotes with escapes) but it would do the job for you.