Richard should have a reason to be suspicious of Mortimer and refusing to capture sir Thomas is most possible reason.
No. But Mortimer was likely present at Radcot Bridge and account of Raphael Holinshed doesn't present him as Richard loyalist:
In 1387, King Richard II sent secretly to Robert de Vere, Duke of Ireland, who was levying troops in Wales, to come to him with all speed, to aid him with the Duke of Gloucester and his friends; and commissioned at the same time Sir Thomas
Molineux de
Cuerdale, Constable of Chester, a man of great influence in Cheshire and Lancashire, and the Sheriff of Chester, to raise troops, and to accompany and safe conduct the Duke of Ireland to the King's presence. Molineux executed his commission with great zeal, imprisoning all who would not join him. Thus was raised an army of 5,000 men. The Duke of Ireland, having with him Molineux, Vernon, and Ratcliffe, rode forward "in statelie and glorious arraie." Supposing that none durst come forth to withstand him. Nevertheless, when he came to
Radcot Bridge, 21 miles from
Chipping Norton , he suddenly espied the army of the lords; and finding that some of his troops refused to fight, he began to wax faint-hearted, and to prepare to escape by flight, in which he succeeded ; but Thomas Molineux determined to fight it out. Nevertheless, when he had fought a little , and perceived it would not avail him to tarry longer, he likewise, as one despairing of the victory, betook himself to flight ; and plunging into the river, it chanced that Sir Roger Mortimer, being present, amongst others, called him to come out of the water to him, threatening to shoot him through with arrows, in the river, if he did not. "If I come," said Molineux,"will ye save my life?" "I will make ye no such promise," replied Sir Roger Mortimer, "but, notwithstanding, either come up, or thou shalt presently die for it." "Well then," said Molineux, "if there be no other remedy, suffer me to come up, and let me try with hand blows, either with you or some other, and so die like a man." But as he came up, the knight caught him by the helmet, plucked it off his head, and straightways drawing his dagger, stroke him into the brains, and so dispatched him. Molineux, a varlet, and a boy were the only slain in the engagement; 800 men fled into the marsh, and were drowned ; the rest were surrounded, stript, and sent home. The Duke of Ireland made his escape to the Continent ; and the King returned to London