  We give an infinite family of monoids $\Pi_N$ (for $N=2, 3, \dots$), each
with a single defining relation of the form $bUa = a$, such that the Dehn
function of $\Pi_N$ is at least exponential. More precisely, we prove that the
Dehn function $\partial_N(n)$ of $\Pi_N$ satisfies $\partial_N(n) \succeq
N^{n/4}$. This answers negatively a question posed by Cain & Maltcev in 2013 on
whether every monoid defined by a single relation of the form $bUa=a$ has
quadratic Dehn function. Finally, by using the decidability of the rational
subset membership problem in the metabelian Baumslag--Solitar groups
$\operatorname{BS}(1,n)$ for all $n \geq 2$, proved recently by Cadilhac,
Chistikov & Zetzsche, we show that each $\Pi_N$ has decidable word problem.
