there recently was a paper about it which I can't find right now

I meant this one: More is Less: On the End-to-End Security of Group Chats in Signal, WhatsApp, and Threema

Here is the abstract:

Abstract—Secure instant messaging is utilized in two variants:
one-to-one communication and group communication. While
the first variant has received much attention lately (Frosch
et al., EuroS&P16; Cohn-Gordon et al., EuroS&P17; Kobeissi
et al., EuroS&P17), little is known about the cryptographic
mechanisms and security guarantees of secure group communication
in instant messaging.
To approach an investigation of group instant messaging
protocols, we first provide a comprehensive and realistic security
model. This model combines security and reliability goals
from various related literature to capture relevant properties
for communication in dynamic groups. Thereby the definitions
consider their satisfiability with respect to the instant delivery
of messages. To show its applicability, we analyze three widely
used real-world protocols: Signal, WhatsApp, and Threema.
Since these protocols and their implementations are mostly
undocumented for the public and two out of three applications
among them are closed source, we describe the group protocols
employed in Signal, WhatsApp, and Threema. By applying
our model, we reveal several shortcomings with respect to
the security definition. Therefore we propose generic countermeasures
to enhance the protocols regarding the required
security and reliability goals. Our systematic analysis reveals
that (1) the communications’ integrity – represented by the
integrity of all exchanged messages – and (2) the groups’
closeness – represented by the members’ ability of managing
the group – are not end-to-end protected.
We additionally show that strong security properties, such
as Future Secrecy which is a core part of the one-to-one
communication in the Signal protocol, do not hold for its group
communication.