Bus arrival base version switching between two values

I was using the request /interfaces/ura/instant_V1?returnlist=baseversion to check if the bus stop data had changed. For example,
[4,“1.0”,1508933955240]
[3,“20170812”]

If the stop data had changed then the version in the base version array, in this case “20170812” would be different from that in the previous request.

This saved downloading ~740kB and was working up until last week when the base version array started switching between two values.

I ran a script which extracted the base version array every hour and printed the result if the version had changed,
previous current
2017-10-24-16-31 [3,“20171020”] [3,“20170812”]
2017-10-24-16-51 [3,“20170812”] [3,“20171020”]
2017-10-24-17-11 [3,“20171020”] [3,“20170812”]
2017-10-24-17-31 [3,“20170812”] [3,“20171020”]
2017-10-24-17-43 [3,“20171020”] [3,“20170812”]
2017-10-24-18-43 [3,“20170812”] [3,“20171020”]
2017-10-24-21-43 [3,“20171020”] [3,“20170812”]
2017-10-24-22-43 [3,“20170812”] [3,“20171020”]
2017-10-24-23-43 [3,“20171020”] [3,“20170812”]

Has anybody got any idea why this is happening ?