Oscification doesn´t trigger the Rotpriest and it is correct.
Read carefully the Ropriest description, it has to be a spell being CASTED TARGETING the rotpriest.
Ossification is casted TARGETING a land, then when it enters only at that moment it targets the rotpriest, meaning that the effect will not trigger.

Every spell that lands first on the battlefield and then targets the rotpriest will not trigger the effect.

opponents ossification targeted my venerated rotpriest but the priests ability didnt trigger. happend multiple times now.

Hello,

For this issue, please submit at: https://mtgarena-support.wizards.com/hc/en-us/requests/new and select 'Report a Problem in your Event'.

Thanks!

When I target my own Venerated Rotpriest with a spell there is no trigger for the opponent to receive a poison counter.

It shouldn't. The spell targets the land, not the Rotpriest. Then the ability targets the Rotpriest, who is not triggered by abilities.

I had Venerated Rotpriest in play & a 1/1 copy of it on the field. My 1/1 copy was targeted by my opponent & neither Venerated Rotpriest triggered.

When Ossification targets, it is not a spell, it is a permanent.