An Caighdeán Oifigiúil only prescribes the synthetic form (the form that contains the pronoun) of the muid form in the past tense, whereas it prescribes both the muid and the mé synthetic forms in the present. (At least I'm pretty sure this is why. To my understanding there is historical and dialectal variation regarding use of synthetic vs. analytic forms.)
The revised version of the Caighdeán (An Caighdeán Athbhreithnithe) accepts the full range of synthetic forms in the Aimsir Chaite. In this case, 'Cheapas gur cheannaíos beoir' is now wholly acceptable.
The revised Caighdeán also allows for the use of the analytic form of 'muid', rather than the synthetic, in the Aimsir Chaite.