@Bapun even I got the same Rs50 CB for BSNL Rs1999 recharge (600GB data without any daily limits, UL calls, SMS for 365 days) today. It is either a glitch or Google trying / persuading people to make recharges using them as many people stopped recharging via GPay & PhonePe when they started charging some convenience fee like thing for them.
One more thing bro, Google / GPay will remind you for another recharge, just days before the expiry of that recharged plan/pack, without fail, in coming days.