Mullin has now been left out of Phil Parkinson's team entirely in four of the club's last five League One games.
This has led to some fans being left in shock after the striker played such a pivotal role in their rise up the Football League.
He fired Wrexham to promotion from the National League and into the EFL before helping secure a place in League One last season.
The 30-year-old has scored over 100 goals since his arrival in 2021 and has become one of the faces of the club.
So much so, that co-owner Ryan Reynolds even cast him as part of the latest Deadpool movie as Welshpool.
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But that fairytale seems to have come to an abrupt stop for Mullin, as he's now found himself on the fringes of the squad.
As mentioned, he's struggled for minutes in the third tier of English football, not appearing at all in any league game since January 28 and only making nine starts all season - scoring three goals.
Mullin came off the bench in midweek to feature in the semi-finals of the EFL trophy against Peterborough United.
However, that was a nightmare appearance, as he missed his penalty in the shootout on the way to losing to Peterborough and ending owners Reynolds and Rob McElhenney's Wembley dream.
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Only days after that penalty miss, Mullin was back out of the squad on Saturday, leaving fans questioning his absence.
One fan wrote on X: "Has Mullin been injured or just dropped the past few league games?"
Another said: "What happened to Ollie Palmer and Paul Mullin?"
"Something's definitely happened between Mullin and Parkinson," a third supporter theorised.
Meanwhile, a fourth wrote: "Mullin not even on the bench, looks like he's on the naughty step for missing his penalty!"
Manager Parkinson appears to prefer his other striker options, and he certainly had plenty of them.
Club-record signing Sam Smith has been starting up front since his arrival in January, along with Jay Rodriguez.
And ex-Premier League forward Steven Fletcher is also part of the strong attacking group, as well as Jack Marriott.
So, it has left Mullin dropping down in the pecking order, and the step to League One may be too much for him.
He had suffered from injury at the start of the campaign, missing multiple games after surgery on his back.
On top of that, he then suffered a shin issue, but he has since recovered and is available to play, as shown in the EFL Trophy.
That means his absence is clearly by choice of the manager, and his future at Wrexham is now being questioned.
Former Red Dragons midfielder Waynne Phillips told the BBC: "For me, there's something not right.
"To not have Paul Mullin in and around the squad seems a little bit crazy. But Phil Parkinson is the manager.
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"He has done particularly well for Wrexham over the last two or three seasons, and you have to accept that he is the man who picks the side."
Wrexham are next in action on Tuesday evening as they face a trip to fellow promotion-chasing side Huddersfield.
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Mullin will hope to be back in the squad for that game, with a win potentially moving his side into the top two.
They currently sit third in League One and two points behind the automatic promotion places.