I also have this same issue.  A single recipient and I am able to send a message (sms/rcs(?)) without issue but for messages intended for multiple users, it appears the message takes upon hitting enter but nothing is sent (nor reflected in the KDS Connect SMS program).  Verified by checking Messages/SMS app on Android phone (LineageOS).

yay -Qi kdeconnect:

Name            : kdeconnect
Version         : 25.04.0-1

Operating System: Arch Linux 
KDE Plasma Version: 6.3.5
KDE Frameworks Version: 6.14.0
Qt Version: 6.9.0
Kernel Version: 6.14.6-arch1-1 (64-bit)
Graphics Platform: Wayland
Processors: 4 × Intel® Core™ i5-4690K CPU @ 3.50GHz
Memory: 19.5 GiB of RAM
Graphics Processor: NVIDIA GeForce GTX 1080

Seeing the same thing - at this point it seems safe to mark this Confirmed?

My phone is a Samsung XCover6 running Android 15

➜  fastfetch  -l none
  ben@t480-ben

󱋩  aurora-dx:stable 🔐
󰣛  Aurora (Version: 42.20250608.1 / FROM Fedora Kinoite 42)
  Linux 6.14.5-300.fc42.x86_64
󰅐  6 days, 15 hours, 49 mins
󰔠  Forged on Jun 06 2025

󰾰  20L6S07G00 (ThinkPad T480)
󰻠  Intel(R) Core(TM) i5-8350U (8) @ 3.60 GHz
󰍛  Intel UHD Graphics 620 @ 1.10 GHz [Integrated]
󰧑  10.65 GiB / 15.49 GiB (69%)
  36.77 GiB / 475.35 GiB (8%) - btrfs [Read-only]
󰍹  1920x1080 @ 60 Hz (as 1536x864) in 14" [Built-in]
  89% [Discharging]

󰕮  KDE Plasma 6.3.5
  KWin (Wayland)
  bash 5.2.37
  Ptyxis 48.4
󰏖  2913 (rpm), 51 (flatpak), 8 (brew)

Same thing, tested all my group sms, sending a text never appears. 

Operating System: EndeavourOS 
KDE Plasma Version: 6.4.1
KDE Frameworks Version: 6.15.0
Qt Version: 6.9.1
Kernel Version: 6.15.4-zen2-1-zen (64-bit)
Graphics Platform: Wayland
Processors: 6 × Intel® Core™ i5-9600K CPU @ 3.70GHz
Memory: 32 GiB of RAM (31.3 GiB usable)
Graphics Processor: AMD Radeon RX 6700 XT

This does not reproduce for me. I am able to send an MMS both to a single target (by setting auto MMS conversion in the KDE Connect Android app settings) and to multiple target.

For those who are impacted, please share more about your Android in particular.

- Phone Vendor (Samsung, Google, etc.)
- Phone Model (Galaxy S12, Pixel 6a, etc.)
- Phone ROM, if not stock
- Phone Android version
- SMS app in use on phone (Google Messages, Samsung Messages, Signal, etc.)
- KDE Connect Android version

This works for my on my OnePlus Fold, stock ROM, Android 15, with Google Messages, and KDE Connect Android 1.32.9.

I see Kan is using CalyxOS; I don't know about that ROM in particular but the Wikipedia page mentions that it is focused on privacy and security, which could indicate that they have protected the user's SMS in a way that prevents KDE Connect from working.

Operating System: Arch Linux 
KDE Plasma Version: 6.4.3
KDE Frameworks Version: 6.16.0
Qt Version: 6.9.1
Kernel Version: 6.15.8-arch1-2 (64-bit)
Graphics Platform: Wayland
Processors: 4 × Intel® Core™ i5-4690K CPU @ 3.50GHz
Memory: 20 GiB of RAM (19.5 GiB usable)
Graphics Processor: NVIDIA GeForce GTX 1080

Phone Vendor:  Google
Phone Model:  Pixel 2 XL
Phone ROM:  LineageOS 22.2 (updated to within a couple weeks)
Android 15
SMS app is Messaging (all permissions)
KDE connect on Android version:  1.33.4

🐛🧹 ⚠️ This bug has been in NEEDSINFO status with no change for at least 15 days. Please provide the requested information, then set the bug status to REPORTED. If there is no change for at least 30 days, it will be automatically closed as RESOLVED WORKSFORME.

For more information about our bug triaging procedures, please read https://community.kde.org/Guidelines_and_HOWTOs/Bug_triaging.

Thank you for helping us make KDE software even better for everyone!

"I see Kan is using CalyxOS; I don't know about that ROM in particular but the Wikipedia page mentions that it is focused on privacy and security, which could indicate that they have protected the user's SMS in a way that prevents KDE Connect from working."

I had this exact same issue on stock Android, same phone, Pixel 4a5G, Android 14.