Defender Scheduled Scans & Definition Updates Not Working Correctly?

Hi,
I am really baffled by the following situation.

On Windows 10 Pro 1803 I set up tasks for scheduled scans and definition updates as follows:

1. Using Group Policy Editor I set up definition updates to run every 3 hours starting at 6am in the morning. The updates occur every 3 hours but not at the times I set up. There are also random, additional updates for the definitions.

2. Using Task Scheduler, I set up to run a quick scan once per day at 6:30pm in the evening. The scan did run but then almost 3 hours later, for no reason, another quick scan was initiated, but no indication as to what triggered this.

I found some older posts online that seem to suggest GPE settings are not being respected but I have no idea why or whether there is something I am missing in all of this.

Has anyone else experienced this?

What possible fixes are there?

Thanks!

Update:

1. The scheduled quick scan is running as I wanted and no additional scans have been initiated.

2. the definition updates are running every 3 hours but it starts at random times. In other words, not at 9am, which would be the first time to check after 6am, but rather at 8:20am (at least this morning).

Since nobody else appears to have encountered this issue and since things are working (more or less), I am marking this as Solved and putting it down to Microsoft/Windows quirks