55 min. is a legal connection in IAH.

ac.com can't or won't price the outbound connection via IAH as OP has found it only shows DEN options.

The outbound is a very straightforward through AC fare YZF-AUS as all connections are within 4 hours and so OW fare (T10Q3TP fare basis) applies.

The inbound is actually two fare basis as the connection in YYC is well over 4 hours so the GDS is pricing it as one OW UA fare basis TA21FN AUS-YYC and a separate AC OW fare (T7I2TPB fare basis) for the YYC-YZF segment.

FWIW a TA with GDS access can ticket this as ITA shows so I'd echo the earlier comment to seek one out. Good luck!