I have been using Meraki equipment for a number of years now and have never seen this. I claimed a licence in February 2021 for 1 MS120-48FP, 1 MX67C and 9 Wireless APs which has become invalidated in June 2021. I contacted Meraki support who have said that the license was invalidated due to an RMA (1 MX67C which was DOA).
I said that I wouldn't expect a RMA to invalidate a licence (I've RMA'd other devices with no license issues in the past) and asked that they replaced or reinstate the license. They responded saying, "Licenses have to get invalidated when they are returned, otherwise they would remain active in a customer's org. The license will need to be replaced with another purchase. Please reach out to the sales rep ... for license purchase info."
Now as far as I am aware I returned a single device due to an RMA. How can I go about getting my 3 year licence - which I have paid for - reactivated?
You can request an RMA due to hardware failure. In this case, you get a replacement device and your licence is unaffected.
You can request an RMA for return (not replacement). This can occur in some cases with trials, with a cancelled order within a specific time frame, etc. This cancels the licence as well. You don't end up with a device.
You get another special category is when you have an older device and it gets RMA'd with a newer device, and the licence has to get changed from the old device to the new. e,g. MX64 ends up getting replaced with an MX67 under RMA.
If you requested an RMA for hardware replacement - then it sounds like the wrong type of RMA was keyed. You should not have gotten a replacement device though. It may be a case of checking the type of RMA requested with support.