Read Only not enforced

I have a 1:M relationship from module PGOpp to Billing Ledger.

PGOpp has an Amount field which should be read only once Ledgers start being related to it

I changed the Amount field to have Read only if, with the formula:

1
greaterThan(count($pgopp_partnershipopportunities_apib_apibillingledger_1),0)
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

greaterThan(count($pgopp_partnershipopportunities_apib_apibillingledger_1),0)

But it's not enforced.

I tried changing the formula to:

1
not(equal(count($pgopp_partnershipopportunities_apib_apibillingledger_1),0))
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

not(equal(count($pgopp_partnershipopportunities_apib_apibillingledger_1),0))

Still not enforced.

Any thoughts on why it would ignore the formula?

Thanks,

Francesca