We have only 1 worker on Odoo.sh and don't experience any problems.  I guess it depends on the number of users and processes accessing the server.

Alright, I was really looking forward to answering this one for the community, as I have seen a lot of similar posts without an actual solution.  For those still struggling, I hope this can be one of the solutions for you and save you a lot of time.

The issue here, is that prior to sending a PO or SO email, the system generates a PDF.  To generate that PDF you need at least two workers configured on your server, and checking the log for any mail connections would be fruitless for this.

Therefore, increasing the number of workers for the server was the solution.

Update your odoo.conf:

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.
.
.
workers = XX
addons_path = ......

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Here is a related article to assist in calculating the appropriate number of workers for your server: https://www.odoo.com/nl_NL/forum/help-1/how-do-i-add-worker-to-odoo-176488

(as I don't have the karma points to add a link, there it is above in plain text)