I don't believe this is supported by a technical reading of the rules, but it's generally recommended that if the bonus action granted to solo monsters can be in effect invalidated due to positioning, it's important that the creature not be permitted to delay its primary turn to be immediately before its secondary turn-doing so means that the party has no ability to respond and thus mitigate the bonus turn. If the bonus turn is designed with mitigation in mind, this means it becomes disproportionately powerful.
Less of an issue with charge and movement inclusive bonus turns, and not one at all with Demogorgon style solo monsters. It should be noted, however, that from a player standpoint, letting a solo monster get both of its turns consecutively can be very dangerous-but it's no different from letting all five enemies move in sequence with one another.
The primary reason why letting all five enemies move in sequence is that typically the way that happens is that the party universally wins initiative-making the chances of one of those five enemies being stunned or reduced to 0 or whatever extremely high before they can even act. Tomorrow, we'll go into greater detail about the dynamics of solo monsters versus the dynamics of standard monsters as it pertains to creature longevity.