I have exactly same issue, with my A50 (South east asia 128mb model) SM-A505F/DS with an Australian SIM card (Boost from Telstra) IMS service is the Messageing App I have not tried Factory reset. Problem completely solved by removing the SIM card - not exactly the optimal solution, but does veriry the source of the issue. the one thing different, is that 24 hours after installing that update about 1 week ago, the problem went away. Now last night the phone battery went flat. Today after recharge the problem has returned.
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