A similar case was seen in MikroTik LoRaWAN gateways and concentrator boards - #37 by fandrit, also for unknown sources. Though it was assumed that LoRaWAN devices were around, once new nearby LoRaWAN devices were actually added, their traffic was received just fine.
So, I’d say, no worries so far!
And an aside:
I guess you know that it would be bad practice to send text. That’s not only true for LoRaWAN, but also for plain LoRa. Also, that Base64 encoded representation decodes to a whopping 178 bytes. Though supported on low SFs, I still feel that’s much more than a typical LoRaWAN packet would/should be.
And of course: as the CRC failed, there’s not much use in trying to decode it. So, the web interface is also giving you false information like “Confirmed Data Up”, “Rejoin-request” and all. (And even for valid LoRa packets, which might still not be LoRaWAN, the web interface might give you the wrong details as it just assumes everything is LoRaWAN.)